Bit errors are introduced when digitized bits are transferred through a communication medium. Correcting these errors is vital to insure reliable communications.
It is well recognized that better decisions can be made about the actual value of a received symbol with the availability of soft information. That is, if it were known to the decoder during error correction what the quality of the signal was when the symbol was received, a better decision could ultimately be made about the true value of the transmitted symbol. Soft information represents a measure of the confidence held in any given symbol detection. Soft-decision decoding operates on a recovery scheme having a number of possible values greater than the number of symbols in the set. Further description of soft-decision decoding may be found in U.S. Patent Application No. 07/422177, filed on Oct. 13, 1989 on behalf of Gerald Labedz et al, titled "Soft Decision Decoding With Channel Equalization."
When utilizing convolutional coding with Viterbi decoding, improved BER (Bit Error Rate) performance was obtained in the past by using soft-decision decoding of BPSK (Binary Phase-Shift Keying) signalling. The Euclidean distance between the transmitted and received signal levels was used as the metric in the maximum-likelihood decision process carried out by the Viterbi algorithm. Further improvement was obtained for trellis coded modulation with Viterbi decoding by using the signal amplitude as channel state information for the soft-decision decoding.
In some applications, such as radio communications, high spectrum efficiency is very desirable. This requires high bit rate in a small bandwidth and is achievable by using multilevel signalling. Thus, in a multilevel system, a symbol represents a plurality of bits. Amplitude/phase modulation techniques increase spectral efficiency at the expense of sensitivity. 16QAM (Quadrature Amplitude Modulation) is such a technique. In this signalling process and other modulations employing amplitude shifting, the received signal strength is a function not only of the channel state, but also the transmitted bit information. Thus, obtaining channel state information from received signal strength for soft-decision decoding is not a simple problem.
In addition, bit or symbol interleaving is a common procedure used to combat burst bit error characteristics that are generated in a fading environment. The interleaving process takes consecutive bits or symbols and rearranges their time-domain order so that any collection of bits or symbols that is combined into another symbol does not suffer as much under burst error conditions. Interleaving, when combined with multilevel signalling, further complicates the problem of obtaining soft-decision/channel state information on a bitwise basis for use in convolutional decoding.
Among the previous solutions, only binary signalling such as BPSK or Quadrature PSK, which is simply BPSK transmitted in quadrature, is addressed. There is no defined way to obtain soft-decision/channel state information on a bit basis for a multilevel signalling system. Therefore, a good process of determining bit-wise or symbol-wise channel state information for soft-decision decoding is required.